Interest in the process for cooling sinter in situ on the sinter machine without first breaking and screening the sinter cake has increased to the point where it is commercially important to improve existing technology in this art. The art of first breaking and screening the sinter cake is old, but plants incorporating this art in some form, which were constructed in times past both in the United States and abroad, have experienced poor operating results.
A typical plant has a long sintering strand on which sintering and cooling are carried out in succession, utilizing separate fan systems. The sintering and the cooling portions of the strand are separated by a dead plate which acts as a seal to isolate one fan system from another fan system.
In accordance with present sintering practice, cooling of the sinter is effected by down draft. The sintering portion includes a hearth layer of screened sinter placed on the grates to protect and partially insulate them. Onto the hearth layer, a layer of sinter mix is deposited and the mix is ignited as it passes under an ignition burner. As the strand carrying the burning sinter passes along, it is subjected to a downdraft of air which enhances down-burning in the sinter mix. The bottom of the fire zone of burning sinter reaches the hearth layer just as the strand reaches the dead plate and crosses into a cooling portion.
The cooling portion in conventional practice provides downdraft of sufficient intensity to cool the sinter cake completely; that is to say, to such an extent that the sinter cake when discharged in broken form can be handled safely by rubber belt conveyors.
Although there are a number of advantages to this type of strand cooling, there are also a number of disadvantages. First, the sinter strand and the building housing the machine must be very long, and in such a long machine the lower or return run performs no useful function. This kind of machine is very costly initially and is costly to service and maintain. Second, a long sintering machine generally does not track properly and this can potentially increase wear and require extra maintenance on machine parts, such as wheels, rails, and the sinter pallets. Third, the downdraft cooling pulls all of the heat in the sinter cake down through the machine pallets. The increased heat on the grate bars and the pallet ribs causes these parts to deteriorate or else requires that they be constructed more ruggedly than they need be if another type of cooling were used. Pertinent prior art includes patents: U.S. Pat. No. 3,172,754 to J. A. Anthes, et al. (Anthes); U.S. Pat. No. 3,166,403 to A. M. Schwarz (Schwarz); and U.S. Pat. No. 2,174,066 to N. Ahlmann (Ahlmann).
U.S. Pat. No. 3,172,754 to Anthes relates to a process and apparatus for heat-hardening green pellets of ore by firing them for use in blast furnaces. Although Anthes uses the term "sintering strand", the object to which he applies such term is a traveling grate. A traveling grate is not a sinter strand on which is placed a mixture of ore and solid fuel which is ignited to produce a hard coherent agglomerated sinter cake.
As described in Anthes, heat indurated pellets, whether or not they contain solid fuel, produce discrete hardened pellets and the process is desirably so controlled that caking or adhesion of the pellets does not occur.
Heat hardening of the pellets is performed by downdraft and the bed of pellets then moves immediately over an updraft cooling zone where the pellets are cooled. Hot air above the cooling zone is conducted back to be recuperated by passing through the downdraft indurating zone.
Anthes does not state the temperature of the pellets when they enter the cooling zone, but it is significant to note that he mentions "highly heated" air, probable way over 1000.degree.F. This air, which is too hot to be handled by a fan, contacts the top of the pellet bed right up to the end of the hardening zone. In Anthes, not only is the average temperature of the bed of hardened pellets high, but also the temperature of the top layer of the bed is high.
This situation is quite different from that in the sinter mix bed as described and claimed in the present invention. The temperature differences, among other things, between Anthes and the present invention are patentably distinct, as is the fact that Anthes relates strictly to indurating pellets of ore and not to indurating a sinter mix.
U.S. Pat. No. 3,166,403 to Schwarz relates to sintering of iron ore, a fluxed iron ore sinter product having a low arsenic content from iron ore containing a relatively high percentage of arsenic. Schwarz shows apparatus for sintering iron ore in a bed that is formed of a plurality of distinct and separable layers. At least two distinct horizontally disposed layers of sintering material are superimposed on a continuously moving sinter grate, one of the layers being an acid, or flux-free iron ore sinter mix, and the other layer being a fluxed iron ore sinter mixture.
The spaced apart upper and lower layers are formed of iron ore sinter feed mixture, each one having a different chemical composition from the other. An intermediary layer of metallurgically useful material is disposed between the upper and lower layers, but it does not fuse with itself or the other layers at the sintering temperatures.
Schwarz deals entirely with sintering; there is no teaching of cooling the sinter cake in the patent. Hence, the teaching and claimed present invention is patentably distinct and unobvious from Schwarz.
U.S. Pat. No. 2,174,066 to Ahlmann relates to sintering or calcining of material such as cement raw materials. On a travelling grate a first layer of recuperative material is spread and over this is a layer of material to be sintered or calcined. The layer to be sintered or calcined contains fuel which is ignited so that it burns in a firing zone, and sinters or calcines the material.
When the burning ceases under the influence of downdraft the sinter is cut off by means of a plough or curved metal plate, leaving only the recuperative layer on the grate. Air passing upwardly through the recuperative layer cools it and the air is heated. The hot air is then directed into a preheating zone, before the ignition or firing zone. In Ahlmann, the sinter cake is not cooled by updraft, but is subject only to downdraft. The downdraft enhances the burning which is completed by the time the sinter cake reaches a dead plate zone wherein it is removed by the plough or curved metal plate.
That Ahlmann does not cool the sinter cake by updraft means is a significant and patentable difference between Ahlmann and the present invention.